310 THE NORTHERN MICROSCOPIST. 

The formula 
RP LL’ Le 
Depth of vision = x Te x Sa) 
shows at once how much the depth of vision may vary by achange 
in the conditions—represented by the various factors in the formula 
—which make up the total effect, important among which, as will 
be seen from the form of the equation, is the refractive index 2 of 
the medium in which the object is mounted. 
(Zo be continued.) 
NOTES ON MOSSES. 
NOVEMBER. 
0" the various families of British Mosses by far the largest, as 
well as the most highly-developed, is the family Hypnacez, or 
Feather Mosses: named from itzvos, sleep, on account of some 
fancied soporiferous property, no doubt arising from its crisp and 
elastic nature when dry, making it a very pleasant bed, and a use- 
ful filling for pillows, &c. 
The squirrel and dormouse, and whole tribes of birds use . 
Hypnums as a material for their nests; and if we shake a tuft of 
moss over a sheet of paper we shall find that it harbours a popula- 
tion we little dream of—elegant little mollusca feeding among the 
branches, with tiny beetles and podure and curious acari hiding 
at the roots. 
According to the London Catalogue the family incudes 21 
genera, numbering 126 species, if we add the genus Thuidium, 
unaccountably placed amongst the Leskeacee ; while the genus 
eu-Hypnum, or true Hypnums, is again divided into 11 sub-genera, 
The character and branching of the stems; the arrangement, 
character, and areolation of the leaves ; also the form, &c., of the 
capsule and its lid, constitute the main features upon which these 
divisions are based ; but in many cases these distinctions are not 
well marked, and require very careful examination under the micro- 
scope to correctly classify them. Taken as a whole, the stems of 
this family of pleurocarpous Mosses are mostly creeping and 
_Caespitose, pinnately or irregularly branched; branches erect, 
spreading, or horizontal; leaves ovate, or ovate-lanceolate, more 
or less acuminate, concave, entire, or serrated ; nerved or nerve- 
less ; nerve very variable in length according to the species ; areolz 
mostly narrow and elongated ; capsules cernuous ; rarely pendul- 
ous, but sometimes nearly erect, ovate or oblong, and tapering at 
the base ; lid conical, or more or less obliquely rostrate ; calyptra 

